refactor!: remove the deprecated securityContext value

`securityContext` was deprecated when the chart split it into a pod-level and a container-level value. It
only ever acted as a fallback for the runtime container: when the container-level value was empty, the
deprecated map was used instead. That fallback silently changed behaviour depending on whether an unrelated
value happened to be set, and it kept a third security-related values path alive next to
`deployment.securityContext` and `deployment.gitea.securityContext`.

With the fallback gone, `gitea.runtimeContainerSecurityContext` was identical to
`gitea.containerSecurityContext`, so the helper was dropped and the Gitea container now reuses the shared
one. A deprecation check fails the render when the removed value is still set, because silently ignoring it
would drop `runAsUser`, `runAsNonRoot` or the capability set and let the container run with weaker
restrictions than intended.

BREAKING CHANGE: `securityContext` no longer exists. Use `deployment.securityContext` for the pod-level and
`deployment.gitea.securityContext` for the container-level security context. Installations that still set
`securityContext` will fail to render unless `checkDeprecation` is set to `false`.

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -1089,7 +1089,6 @@ To comply with the Gitea helm chart definition of the digest parameter, a "custo
 | --------------------- | ------------------------------------------------------------------------------------------------------------------------------------ | ----- |
 | `openshift.enabled`   | Enable OpenShift compatibility defaults for chart-managed pods. Defaults to auto-detect based on the SecurityContextConstraints API. | `nil` |
 | `openshift.hostUsers` | Override the PodSpec hostUsers field for chart-managed pods. When unset, the field is omitted so the platform default is used.       | `nil` |
-| `securityContext`     | Run init and Gitea containers as a specific securityContext                                                                          | `{}`  |
 | `podDisruptionBudget` | Pod disruption budget                                                                                                                | `{}`  |
 
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